This invention relates to the field of determination of the volume of particles in suspension in a liquid, and particularly, checking of the functioning of liquid clarification installations. The present invention relates more particularly to a method of and apparatus for measuring the volume of particles of less than a predetermined size and application of this method, among other uses, to automatic measuring of the quality of a purified water, making it possible to determine the optimal amounts of a flocculating agent or agents use for coagulation-flocculation treatment of a waste water.
In numerous industries, for example, metallurgical, chemical, pharmaceutical, agriculture and food industries, or again in pollution control, a great number of techniques are used relating to dissolving, precipitation or flocculation of suspended solid particles. These techniques often require a measurement of the volume of materials suspended in a liquid.
It is known, for example, that in methods of clarifying liquids containing particles in suspension, which are too fine to be filtered or settled, it is generally necessary to provide a flocculating stage or system in which, by use of a suitable flocculating agent, as for example aluminum sulfate or ferric chloride in the case of water. Still, it is practically impossible to provide the exact dose of agent to be added and, if the physical and chemical characteristics of the liquid to be treated (for example, waste water) change over time, particularly in the case of industrial discharges or a dilution resulting from a storm, the dose initially selected for the flocculating agent is no longer suitable and the entire treatment is consequently compromised.
To mitigate the above-mentioned drawback, various techniques and apparatuses have been used or conceived. For example, methods have been proposed making possible the automatic delivery of the optimal dosage of the flocclating agent to be added to the raw water to be treated, thus replacing the laboratory testing, using a test known as th jar test.
In one known device, the turbidity of water is measured by producing signals which regulate the dosage of flocculating agent, based on a relation of correspondence between an increase of turbidity in the phase of floc formation and residual turbidity after settling. Several investigators have shown experimentally that there is a non-constant lag between the dose of reagent that gives maximum turbidity to the flocculated liquid and that which, after flocculation-settling, gives the most clarified liquid; this lag in practice causes an overdose of agent, a distinct shortcoming.
According to another technique, it has been proposed to check flocculation and automatically adjust the necessary amount of flocculating agent by optically measuring the volume of flocs by diffraction of a light beam produced by a laser. Use of a laser signal brings progress in comparison with the turbidimeter such as the one cited because the signal takes into account the different size of the particles and is insensitive to the color of the water. However, study of flocs does not constitute the most advantageous parameter for checking of flocculation and, moreover, this known technique does not make it possible to measure the volume of the materials in suspension in the liquid whose granulometry is less than a predetermined value.